1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic printer, and a safety circuit mounted on the same apparatus.
2. Description of the Related Art
Image forming apparatuses using an electrophotographic recording technique, such as a copier and a printer, are equipped with a fixing apparatus as a fixing unit that heats an unfixed toner image formed on a recording material to fix the image on the recording material. A configuration of a fixing apparatus has been known that includes a heater member (ceramic heater) including a substrate and a heat-generating member (heat-generating resistor) formed on the substrate, and performs heating by causing the heat-generating member to generate heat using power supplied from a commercial power supply. The image forming apparatus is equipped with a safety circuit. In case where a failure brings the heater member into a thermal runaway state, the safety circuit detects overcurrent supplied to the heater member, and stops supplying power to the heater member (Japanese Patent Application Laid-Open No. 2007-212503).
In case where failure brings a fixing apparatus including multiple heat-generating members into a thermal runaway state of supplying power only to one or some of the multiple heat-generating members, stress generated in the heater member (substrate) by the heat of the heat-generating member sometimes becomes higher than the stress during a normal operation. It can be considered that the increase in stress generated in a heater member may break the heater member before the safety element, such as a thermal SW, detects an excessive temperature rise state. To address this problem, measures have been required that adopt a highly responsive temperature detecting element, and adopt a substrate material having a high thermal conductivity to reduce the stress generated in the heater member.